The present invention relates to a laser processing method for glass substrates, and a diffraction grating and a microlens array which can be obtained therefrom.
Silicate glass composed primary of SiO2 is highly transparent and can easily be molded (deformed) at high temperatures. Sheets of silicate glass, which have been formed with holes or concavities and convexities by microscopic topographic processing, are widely used as glass substrates for optical components used for optical communications and display devices.
In order to make a hole in a sheet of silicate glass according to microscopic topographic processing, it has been the general practice to process the sheet of silicate glass with wet etching (chemical etching) using an etchant of hydrofluoric acid or the like, or dry etching (physical etching) such as reactive ion etching.
However, wet etching suffers problems with respect to management and processing of the etchant. Dry etching requires pieces of equipment such as a vacuum container, needs a large-scale apparatus, and is not efficient because a pattern mask has to be produced by complex photolithography.
Laser beams have an intensive energy, and have heretofore been used to increase the temperature of a surface of a material to which the laser beam is applied thereby to ablate or evaporate a portion of the material to which the laser beam is applied, for processing the material in various ways. Since the laser beam can be focused into a very small spot, it is suitable for microscopic topographic processing of a material.
Then, in Japanese Patent Laid-Open No. 54-28590 (1979), there is disclosed processing of a glass substrate surface by radiating it with a laser beam while moving a table in X-Y directions, on which table is fixedly mounted the glass substrate already heated to 300 through 700xc2x0 C.
Although, as mentioned above, the concavities and convexities of desired shape can be formed on the glass surface by moving the table in X-Y directions, the concavities and convexities cannot be created if it is for instance a microscopic pattern such as of a diffraction grating.
Moreover, the movement of the table generates dust, which results in defects in the products and decreases productivity thereof.
As another method of manufacturing a planar microlens array etc., a stamper method has been already known, in which lens material is injected into a mold frame and the molded patterns are transplanted on the glass substrate and baked, however, it requires accurate positioning during the pattern transplanting process and the baking process, and it takes time.
As another method of manufacturing a planar microlens array etc., it has been proposed to obtain a convex lens by forming concavities arc shaped in cross-section on the glass substrate surface with a wet etching and injecting plastic material of high refractive index into the formed concavities, thereby forming the convex lens with the concavities, however, the wet etching has the problems as mentioned above.
Then, it is conceivable to form the concavities into which the plastic of high refractive index is injected by radiating a laser beam through a mask, however, since the laser beam has tendency of going straightforward and it has almost same intensity within area of one spot after passing through the openings of the mask, then the wall of the concavity formed on the glass substrate comes to be about perpendicular to the glass substrate, whereby it is impossible to obtain the cross-section of perfectly continuous arc shape. Therefore, it cannot be mounted onto apparatus requiring extremely high accuracy, such as a liquid crystal display, as it is, and it needs more or less treatment by wet etching and takes time.
Laser beams are generated by an infrared laser such as a CO2 laser, a Nd:YAG laser, a laser comprising aNd:YAG laser combined with a wavelength conversion capability for producing a laser beam whose wavelength ranges from a near-infrared region through a visible region to an ultraviolet region, and an ultraviolet laser such as an excimer laser such as an ArF or KrF laser. If the CO2 laser of long wavelength is used, cracking due to thermal strain occurs violently. If the ultraviolet KrF laser (wavelength of 248 nm) is used, cracking occurs around the area where the laser beam is applied, therefore it is not suitable for the microscopic topographic processing. Thus, the use of the ArF excimer laser of wavelength of 193 nm is optimum as the laser beam for glass processing, however, even when such an ArF excimer laser is used, because of absorption by air, it is needed to replace the air with absorption-free gas such as Ar, etc. or to keep a vacuum in order to allow the laser beam to reach as far away as possible.
The present invention has been made to resolve the conventional problems mentioned above, and an object thereof is to provide a laser processing method able to form microscopic concave patterns on a glass substrate surface with accuracy and within a short time period.
Another object thereof is to provide a laser processing method to form a large number of the concavities having a curved line cross section on the glass substrate surface.
Further another object thereof is to provide a laser processing method to form a large number of the concavities on the glass substrate surface without movement of the glass substrate and by changing the light path.
Furthermore another object thereof is to obtain a diffraction grating and a microlens array in accordance with the above method.
For achieving the object mentioned above, according to the present invention, a laser processing method for a glass substrate comprises: radiating the laser beam on the glass substrate, absorbing energy of the laser beam into the glass substrate, and removing the glass by melting, evaporation or ablation due to the energy, wherein microscopic concavities and convexities are formed on a surface of the glass substrate, by partially varying the spacial distribution of the intensity of the laser beam applied upon the surface of the glass substrate, thereby removing a greater amount of glass where the intensity is stronger, and less where the intensity is weaker.
A diffraction grating or a microlens array which can be incorporated into an optical coupler, a polariscope, a spectroscope, a reflector or a mode transducer, etc., can be manufactured by using a laser beam having periodical or regular distribution in intensity.
The laser beam having the regular intensity distribution can be obtained by a phase mask or interference between two laser beam, and the periodical cross-sectional configuration of the concavities and convexities formed on the surface of the glass substrate can be controlled by the pulse energy of the laser beam. And, the laser beam having the regular intensity distribution can be obtained by using a mesh-like mask, etc.
For achieving the another object mentioned above, according to the present invention, a laser processing method for a glass substrate comprises: disposing a mask at the focus position on the incident side of a lens, disposing the glass substrate at the focus position on the exit side of said lens, radiating the laser beam on the mask thereby forming a Fourier transform image on a surface of said glass substrate at the focus position of the exit side of said lens, absorbing energy of the Fourier transform image into the glass substrate, and removing the glass by melting, evaporation or ablation due to the energy, thereby forming a plurality of concavities periodically distributed on said glass substrate.
Here, the laser beam penetrating the openings of the mask shows a rectanglar intensity distribution in which the intensity is nearly equal at the central and the peripheral portions. However, the Fourier image of the laser beam penetrating said mask shows a sinusoidal intensity distribution which has greater value at the central portion and a lesser value on the peripheral portion thereof. As the result of this, it is possible to form a number of concavities spreading on the surface of the glass substrate in two dimensions, with smoothly curved lines including arc lines in the cross sectional view. For example, applying it to a planar microlens array, it is possible to form a convex lens with high accuracy.
Similarly for achieving the another object mentioned above, according to the present invention, a laser processing method for a glass substrate comprises: coinciding the focal point on the exit side of a first lens with the focal point on the incident side of a second lens, disposing a first mask at the focal point on the incident side of said first lens, disposing a second mask at focal point on the exit side of said first lens, disposing a glass substrate at the focal point on the exit side of said second lens, radiating the laser beam on the first mask thereby forming a Fourier transform image at the focal point on the exit side of said first lens as well as forming a part of a Fourier transform image on a surface of said glass substrate disposed at the focal point on the exit side of said second lens, absorbing energy of the formed image into the glass substrate, and removing the glass by melting, evaporation or ablation due to the energy, thereby forming a plurality of concavities periodically distributed on said glass substrate.
The pattern of the concavities formed on the glass substrate surface by such a method is coincident with that of the first mask, but the cross-sectional configuration thereof is curved smoothly. And, the power of the image can be adjusted by changing the focal length of the two lenses.
Here, the Fourier transform image is formed on the glass substrate by disposing the glass substrate at the focal point on the exit side of the lens, however, according to the present invention, it is also possible to dispose the glass substrate away from the focal point. In this case, not the Fourier transform image, but a periodical structure differing from that of the mask is transferred.
As the masks, not only are a mask having openings, such as a copper sheet (a copper mesh) on which are arranged rectangular or circular holes in two dimensions, and a mask obtained from patterning of layers by metal evaporation on a fused quartz substrate applicable, but also a mask of so called phase type, which gives phase shift to the beam is applicable.
Further for achieving the another object mentioned above, according to the present invention, a laser processing method for a glass substrate comprises: radiating the laser beam on the glass substrate, absorbing energy of the laser beam into the glass substrate, and removing a part of the glass by melting, evaporation or ablation due to the energy, wherein microscopic concavities are formed on a surface of said glass substrate by changing the optical path of the laser beam with optical path changing means, thereby moving a spot position of the laser beam radiated on the surface of said glass substrate.
Here, the optical path changing means can be constructed with a first mirror for moving the spot position of the laser beam in a X-direction on the surface of said glass substrate, and a second mirror for moving the spot position of the laser beam in a Y-direction on the surface of said glass substrate. For the mirrors, it is preferable to use a galvano mirror which turns through a small amount of angle depending on the current conducting through it.
Furthermore, in the conventional art, the laser beam which is applicable to glass processing is limited to an ArF excimer laser of wavelength of 193 nm, and the device is big and complicated because of the necessity of replacement with non-absorbing gas, such as Ar or vacuum. However, according to the present invention, it is experimentally ascertained that a laser beam having a wavelength longer than the above-mentioned is applicable to glass processing, by introducing silver into the glass in the form of Ag atoms, Ag colloid or Ag ions, without cracking or breakage, and the trace of the laser radiation is very smooth.
However, in case that the glass contains silver in uniform concentration, such as the conventional light sensitive glass and/or antibacterial glass, no increase in processability can be found, therefore, it is necessary that it has a concentration slope of the silver showing the highest concentration at a side surface to be processed and gradually decreasing to the other side surface thereor.
This is according to the mechanism shown in FIG. 1 and will be explained below.
As shown in FIG. 1(a), the laser beam is applied onto the surface having the highest Ag ion concentration. Then, as shown in FIG. 1(b), the Ag ion is resolved to be a colloid (very fine particles of Ag) on the surface having the highest Ag ion concentration of the glass substrate. The Ag colloid particles absorb energy of the laser beam, as shown in FIG. 1(c), and melting, evaporation or ablation occurs, whereby the glass of the surface layer is removed. After removing the glass of the surface layer, the same phenomenon occurs in subsequent glass layer, and concavities or penetrating holes are formed at the last as shown in FIG. 1(d).
In this way, since the glass is gradually removed from the top surface of the glass substrate, therefore cracking or breakage is hard to occur. On the contrary to this, in the glass substrate containing silver in uniform concentration or no silver, ablation occurs inside of the glass substrate, and therefore cracking or breakage occur easily.